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AN4450
Application note
Hardware and software guidelines for use of LPS25H pressure
sensor
By Raffaele Di Vaio
Introduction
The LPS25H is an ultra-compact absolute piezoresistive pressure sensor with enhanced
digital features in a small package footprint.
Unless specifically noted, all recommendations in this document apply only to the LPS25H.
Please refer to the LPS25H datasheet (available at www.st.com) for device and feature
definitions.
April 2014
DocID025978 Rev 1
1/26
www.st.com
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Related Manuals for ST LPS25H
Summary of Contents for ST LPS25H
- Page 1 The LPS25H is an ultra-compact absolute piezoresistive pressure sensor with enhanced digital features in a small package footprint. Unless specifically noted, all recommendations in this document apply only to the LPS25H. Please refer to the LPS25H datasheet (available at www.st.com) for device and feature definitions.
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Page 2: Table Of Contents
Hardware (designing PCB schematics and layout) ....6 LPS25H device package, interconnect and polarization ....6 2.1.1... - Page 3 One-shot mode conversion time estimation ..... . . 22 Reference SW to get started with LPS25H ..... . . 22 Pressure to altitude conversion .
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Page 4: Pressure Sensor Evaluation Boards
Figure 2. STEVAL-MKI142V1 - LPS25H adapter The STEVAL-MKI142V1 adapter board is designed to facilitate the demonstration of the LPS25H product. The board offers an effective solution for fast system prototyping and device evaluation directly within the user’s own application. The STEVAL-MKI142V1 can be plugged into a standard DIL 24 socket. The adapter provides the complete LPS25H pin-out and comes ready-to-use with the recommended decoupling capacitors on the VDD power supply line. - Page 5 AN4450 Pressure sensor evaluation boards The STEVAL-MKI109V2 MEMS (aka eMotion board, shipped with pre-loaded eMotion FW) is completed by a MS Windows™-based SW application called Unico. This tool may be used as a simple real-time demonstrator or to verify device performance. It also allows easy monitoring of the register status and allows changes to them based on the intended scenario.
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Page 6: Hardware (Designing Pcb Schematics And Layout)
Hardware (designing PCB schematics and layout) LPS25H device package, interconnect and polarization 2.1.1 Package drawings Pressure sensor LPS25H is available in a holed LGA package: HCLGA-10L (1mm thickness). Figure 3. HCLGA-10L 2.5x2.5x1.0 mm Figure 4. Pin mapping (bottom view) 6/26... -
Page 7: Pin Mapping
AN4450 Hardware (designing PCB schematics and layout) 2.1.2 Pin mapping Table 1. LPS25H pin mapping details Pin # Name Function What to do VDD_IO Power supply for I/O pins I/O supply voltage (1.7V ~ 3.6V) Optional pull-up resistor, connected to VDD_IO, I²C serial clock (SCL) -
Page 8: Typical Application Circuit
VDD. In the example shown here, VDD_IO is connected to VDD. Please note that the LPS25H does not have an internal pull-up on the I²C lines (SCL and SDA). They must be added externally according to the I²C bus speed and load, and connected to VDD_IO. -
Page 9: Pressure Sensor Pcb Layout And Soldering Recommendations
(CTRL_REG2[3]=1). Pressure sensor PCB layout and soldering recommendations The LPS25H has an aperture on top of the package, so special care is required since sensor performance could be compromised by: Mechanical stress coming from the PCB board –... -
Page 10: Stencil Design And Solder Paste Application
Hardware (designing PCB schematics and layout) AN4450 Figure 6. Recommended land and solder mask design for *LGA packages ≥ A = Clearance from PCB land edge to solder mask opening 0.25 mm to ensure that solder mask is opened externally to device area ... -
Page 11: Manual Soldering
No need to use any tape to cover the hole on the CAP of the LPS25H during reflow. The cleaning of the PCB is not a general task, as it should be cleaned only when the PCB is contaminated during reflow. -
Page 12: Power Supply: Consumption Estimation And Optimization
The operating voltage for both VDD_IO and VDD is 1.7 V to 3.6 V. The LPS25H can also be powered directly by a small lithium coin battery like the CR2032. This type of battery has a current capability of 220 mAh, enough to sustain the sensor peak current (less than 1 mA). - Page 13 – Idd= 3.6 µA In order to improve system power saving, the LPS25H embeds a FIFO buffer capable of storing up to 32 pressure output values. Since the host processor does not need to continuously poll data from the sensor, it can wake up (on programmable interrupt from the sensor) only when requested and burst the significant data out from the buffer.
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Page 14: Common Rules For Low Power Consumption
Power supply: consumption estimation and optimization AN4450 Common rules for low power consumption Here are some notes about possible sources of power leakage: – Do not leave any input pin un-terminated (floating). This may leave the internal circuit in undefined state, creating/introducing extra power consumption. Connect unused input pins (such as CS, and SA0 if a single sensor is used on the I C bus) to VDD_IO. -
Page 15: Using The Device Step-By-Step, From Basic To Advanced
AN4450 Using the device step-by-step, from basic to advanced Using the device step-by-step, from basic to advanced First time bring-up (I²C example) Start by using the same supply for VDD_IO and VDD to check device functionality. Check supply impedances and ensure that all the pins voltages are in static condition. The device is a slave with 1 byte sub-address which MSB should be ‘1’... -
Page 16: One-Shot Mode Measurement Sequence
(see Section 6.7). In special cases, there’s also the possibility to store the RPDS calibration value inside the LPS25H non-volatile memory. 16/26 DocID025978 Rev 1... -
Page 17: Using Fifo Modes
Using FIFO modes Effective use of the FIFO The LPS25H embeds a 32-slot x 24 bit FIFO for pressure data coming from the registers that normally feed PRESS_OUT (@ 0x28/0x29/0x2A). It allows lower frequency of serial bus transactions and provides more time to collect all measurements taken. -
Page 18: Bypass Mode (F_Mode[2
Using FIFO modes AN4450 The whole FIFO content can be read by reading 3x32 bytes from PRESS_OUT_XL location in a single I²C read transaction. Internally the reading address will automatically roll back from 0x2A down to 0x28 when FIFO is active to allow a quick read of its content. 5.1.2 Bypass mode (F_MODE[2..0]="000"... -
Page 19: Hardware Digital Filter
AN4450 Using FIFO modes Table 3. Number of averaged samples vs WTM_POINT setting WTM_POINT[4..0] Number of averaged samples 00001 00011 00111 01111 11111 While the speed of new averaged output is the one defined by ODR[2..0] in register CTRL_REG1 (0x20). 25Hz for ODR=4 12.5Hz... -
Page 20: Extra Fifo Modes
Using FIFO modes AN4450 Figure 9. Hardware digital filter The digital filter reduces the pressure noise level to 0.010 hPa rms (1 Pa at 1 sigma) and allows to reduce the internal ADC HW average, reducing power consumption while keeping the same pressure noise level. -
Page 21: Bypass To Stream Mode (F_Mode[2
AN4450 Using FIFO modes Extra FIFO modes For all the following modes the trigger signal is the IA bit[2] of register INT_SOURCE (0x25) which is configured by register INTERRUPT_CFG (0x24). 5.2.1 Stream to FIFO mode (F_MODE[2..0]="011" in FIFO_CTRL (0x2E)) The FIFO works in Stream mode till a trigger event occurs, then it changes to FIFO mode. 5.2.2 Bypass to Stream mode (F_MODE[2..0]="100"... -
Page 22: Software
AN4450 Software A detailed register list is reported in the LPS25H datasheet. In This section we collect useful information on how to handle possible exceptions on the communication bus and how to properly use the information coming from the device. -
Page 23: Sw Filtering & Internal Fifo
AN4450 Software //=feet/3.280839895 *m = *ft/3.280839895; Terminology is very delicate for altitude. There is not one definition of altitude: Absolute pressure sensor measures the air pressure at the sensing point. The base rule of thumb is that the pressure drops by 1 hPa every 8.3 meters. The “sea level pressure average”... -
Page 24: Absolute Accuracy Drift Due To Soldering
Software AN4450 Follow this sequence: Turn on BOOT bit (bit[7] =1 of CTRL_REG2 (0x21)) Wait for the BOOT bit to reset (BOOT bit is self-clearing, it is set again to '0' by hardware at the end of the process) Wait an additional 5 ms Configure the registers Power on the device for normal use Absolute accuracy drift due to soldering... -
Page 25: Revision History
AN4450 Revision history Revision history Table 5. Document revision history Date Revision Changes 29-Apr-2014 Initial release. DocID025978 Rev 1 25/26... - Page 26 No license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. If any part of this document refers to any third party products or services it shall not be deemed a license grant by ST for the use of such third party products or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoever of such third party products or services or any intellectual property contained therein.
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